In both animal proof of concept studies and preliminary clinical trials, there is ample data demonstrating the potential therapeutic benefits o CD154 blockade for treatment of GVHD, organ transplantation and autoimmune diseases. However, development of CD154 as a therapeutic has been impeded by antibody toxicity observed in early clinical trials. GVHD is a complication of allogeneic Hematopoietic Stem Cell Transplantation (HSCT). GVHD remains a major cause of mortality in approximately 50% of patients who survive > 1 year post transplant. The standard of care is limited to drugs that need to be taken long term, work moderately well and are associated with significant side effects. There is therefore profound unmet need and significant potential for drugs that are safe and efficacious. Studies in GVHD have demonstrated that anti-CD154 acts as a prophylactic and is effective as a monotherapy as demonstrated in NHP where permanent allograft tolerance can be achieved using short courses of treatments comprised of ?CD154 alone. This strategy eliminates the use of steroids and calcineurin inhibitors both of which are associated with numerous side effects. ?CD154 thus has a competitive advantage as most other drugs in development will require some form of combination therapy with either steroids or calcineurin inhibitors. A similar opportunity for improved clinical outcomes due to effective induction of tolerance exists in recipients of solid organ transplants. Furthermore, virtually all autoimmune disease models can be effectively ameliorated with ?CD154 therapy, with long-term remission observed. We will target GVHD and chronic rejection associated with transplantation as our first clinical indication for commercial development. Existing studies strongly suggest that domains within the Fc region of the ?CD154 mAb contribute to its toxicity and therapeutic capacity. When toxicity was observed in the clinic and retrospectively in NHP, modifications were made to the antibody; while these modifications eliminated toxicity in NHP, the efficacy of ?CD154 as a tolerogenic antibody also was significantly reduced. As a result, development programs for ?CD154 as a therapeutic stalled. In Phase 1 studies we identified modifications that resulted in safe and efficacious versions of CD154 as tested in murine models. The goal of this proposal is to build on those observations and generate variant forms of the ?human CD154 antibody that retain the beneficial tolerogenic effects of ?CD154 while greatly reducing or eliminating toxicity. Variant forms of the antibody will be evaluated for both safety and efficacy in NHP models. Successful proof of concept in NHP transplant models will be the basis for creating a novel therapeutic, which could have far-reaching impacts on the treatment of autoimmune diseases and organ transplantation.